In an integrated circuit, a signal can couple from one node to another via the substrate. This phenomenon is referred to as substrate coupling or substrate noise coupling. Moreover, a substrate that is susceptible to substrate noise coupling may be described as having a low insertion loss, where insertion loss is a decrease in transmitted signal. Substrate noise coupling remains one of the main concerns in low noise circuits for mixed signal and system-on-chip (SOC) designs.
The push for reduced cost, more compact integrated circuit systems, and added customer features has provided incentives for the inclusion of analog functions on primarily digital integrated circuits (ICs) forming mixed-signal ICs. In these systems, the speed of digital circuits is constantly increasing, chips are becoming more densely packed, interconnect layers are added, and analog resolution is increased. In addition, recent increases in wireless applications and its growing market are introducing a new set of aggressive design goals for realizing mixed-signal systems.
However, in mixed-signal systems, both sensitive analog circuits and high-swing, high-frequency noise injector digital circuits may be present on the same chip, leading to undesired signal coupling between these two types of circuits via the conductive substrate. Additionally, the reduced distance between these circuits, which is the result of constant technology scaling, exacerbates the noise coupling.
A challenging task, applicable to any mixed-signal IC, is to minimize noise coupling between various parts of the system to avoid any malfunctioning of the system. In other words, for successful system-on-chip integration of mixed-signal systems, the noise coupling caused by non-ideal isolation should be minimized so that sensitive analog circuits and noisy digital circuits can effectively coexist, and the system operates correctly.
The primary mixed-signal noise coupling problem comes from fast-changing digital signals coupling to sensitive analog nodes. Another significant cause of undesired signal coupling is the cross-talk between analog nodes themselves owing to high-frequency/high-power analog signals. One of the media through which mixed-signal noise coupling occurs is the substrate. Digital operations cause fluctuations in the underlying substrate voltage, which spreads through the common substrate causing variations in the substrate potential of sensitive devices in the analog section. Similarly, in the case of cross talk between analog nodes, a signal can couple from one node to another via the substrate.
Additionally, substrate noise coupling is a concern with low noise amplifiers (LNAs). A LNA is a special type of electronic amplifier or amplifier used in communications systems to amplify very weak signals captured by an antenna, e.g., of a radio frequency telescope. The LNA may be located close to the antenna, such that the losses in the signal path become less critical. Using a LNA, the noise of all subsequent stages is reduced by the gain of the LNA and the noise of the LNA is injected directly into the received signal. Thus, it is desirable for a LNA to boost the desired signal power while adding as little noise and distortion as possible so that the retrieval of this signal is possible in the later stages in the system.
Furthermore, substrate noise coupling is a concern with phase-locked loop (PLL) systems. A PLL is an electronic control system that generates a signal that is locked to the phase of an input or “reference” signal. This circuit compares the phase of a controlled oscillator to the reference, automatically raising or lowering the frequency of the oscillator until its phase (and therefore frequency) is matched to that of the reference. Phase-lock loops are widely used in radio, telecommunications, computers and other electronic applications to generate stable frequencies, or to recover a signal from a noisy communication channel. Since a single integrated circuit can provide a complete phase-lock-loop building block, the technique is widely used in modern electronic devices, with output frequencies from a fraction of a cycle per second up to many gigahertz. However, because the PLL is formed on a single integrated circuit, the device is susceptible to substrate noise coupling. For example, in a cellular phone transceiver, when the pre-driver operates, the PLL output becomes very noisy due to coupling with the substrate.
Conventionally, in attempting to minimize substrate noise coupling, designers have used deep and shallow trench isolations, guard ring structures and high doping layer/triple well structures. However, each of these noise isolation techniques suffer from drawbacks.
For example, deep and shallow trench isolations are too shallow and have no bottom, and thus cannot completely isolate substrate noise or eliminate substrate noise coupling. More specifically, deep trench isolations are typically 3 to 10 microns in depth and shallow trench isolations are typically 0.3 to 2 microns in depth. However, the depth of a substrate in which these deep or shallow trench isolations may be formed is typically at least 250 microns. Thus, noise has enough path in the substrate to bypass the deep and/or shallow trench isolations.
Moreover, guardrings, which are a type of trench isolation, are typically made of metal and formed by lithography and etch processing. However, these techniques may have the low insertion loss at high frequencies due to the parasitic capacitance and the shallow depth, which may render the device unsuitable for high frequency operations.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.